virtio_bt: call scheduler when we free unused buffs

For virtio-net we were getting CPU stall warnings, and fixed it by
calling the scheduler: see f8bb5104 ("virtio_net: suppress cpu stall
when free_unused_bufs").

This driver is similar so theoretically the same logic applies.

Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
Message-Id: <20230609131817.712867-4-xianting.tian@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>